Frequency discriminator circuits are used in a myriad of applications wherein it is necessary to distinguish the frequency of a data input signal about a predetermined threshold. For example in an application involving the control of the servo mechanism of a magnetic disk drive, the frequency of the data input signal must be differentiated either above or below the threshold a reference frequency. A common prior art technique for frequency discrimination uses a D-type flipflop in combination with a timing capacitor and comparator for monitoring the voltage across the timing capacitor against a reference potential. The output of the comparator is coupled to the data input of a second D-type flipflop while the data input signal is applied at the clock input of the same for providing an output signal to indicate whether the frequency of the input signal is above or below a predetermined threshold. A logic one is applied at the data input of the D-type flipflop, while the data input signal is applied at the clock input of the same. The output signal of the D-type flipflop is applied back to the reset input yielding narrow pulses at its Q-output. The narrow pulses charge the timing capacitor while a current source continuously discharges the same. The triangular voltage waveform developed across the timing capacitor is compared to the reference potential for providing a high output signal if the voltage across the timing capacitor is greater than the reference potential and a low output signal when the timing capacitor voltage is less than the reference potential.
If the frequency of the data input signal is greater than the predetermined threshold, the narrow pulses appearing at the Q-output of the D-type flipflop circuit repeat at a sufficiently rapid rate to maintain the voltage across the timing capacitor above the reference potential and keep the output signal high. Otherwise if the frequency of the data input signal is low, the longer intervals between the output pulses of the D-type flipflop allows the current source to discharge the timing capacitor below the reference potential. The output signal of the frequency discriminator circuit drops to logic zero indicating that the frequency of the data input signal is below the predetermined threshold as determined by several analog control parameters including the value of the capacitor, the magnitude of current flowing through the current source and the reference potential.
Unfortunately, the timing capacitor and current source are temperature dependent devices and subject to manufacturing process variation, while the reference potential often contains external noise. A steady operating point is difficult to maintain for the predetermined frequency threshold as the aforedescribed variations in the analog control parameters limit the accuracy thereof and thus the resolution of the frequency discriminator circuit. This is especially true when attempting to track the data input signal over a range of frequencies. It is desirable to eliminate the analog components such as the timing capacitor, current source and external reference potential.
Hence, there is a need for an improved frequency discriminator circuit having greater accuracy in distinguishing the frequency of the data input signal about a predetermined threshold.